160 



EARTH FEATURES AND THEIR MEANING 



tables which are separated by canons (Fig. 166). At the same 

 time the grade of the channel is becoming natter, and its precipi- 

 tous walls are being replaced by curving slopes, as will be more 



FIG. 165. Two successive forms of gullies from the earliest stage of a 

 river's life (after Salisbury and Atwood). 



fully described in the sequel. It is because of this progressive 

 reduction of grades with increasing age that the early stages of 

 a river's life are much the most turbulent of its history. The 



FIG. 166. Partially dissected upland (after Salisbury and 

 Atwood). 



water then rushes down the steep grades in rapids, and is often 

 at times opened out in some basin to form a lake where differ- 

 ences of uplift have been characteristic of neighboring sections. 



